Contractile proteins from skeletal muscle and cardiac muscle will be studied in vitro, principally using fluorescent probes. The contractile proteins tropomysin, actin and troponin will be labeled mainly using -SH reactive fluorescent probes. The localized environment and protein-protein binding will be studied under various physiologically important conditions, including the absence and presence of calcium ions, in order to evaluate changes in confomation that accompany activation of the contractile process. Donor/acceptor pairs will be used to study the spatial organization of the proteins by means of fluorescence energy transfer. Distances between probes attached to specific pairs of sites will be determined under various physiological conditions in order to evaluate movement of thin filament proteins that accompany activation of the contractile process.